This invention relates to a spin control system for vehicle wheels.
When driving power is applied to the driven wheels of a vehicle, a tractive force between the wheel and the road surface is generated that is dependent upon various parameters which include road surface conditions and the amount of slip between the wheel and the road surface. Slip is defined as the difference between vehicle speed and wheel speed divided by vehicle speed. Thus, when wheel speed is greater than vehicle speed, slip is negative. The wheel/road tractive force increases as wheel slip decreases (i.e. wheel slip becomes more negative), until a critical value of slip is surpassed. Beyond this critical slip value, the tractive force decreases and the wheel rapidly approaches free spin. If the wheel is allowed to freely spin, the vehicle will not accelerate. Thus, optimal vehicle launch occurs when the wheel slip is at or near the critical slip value for the particular road surface.
A wheel spin control system regulates either one or both engine power and brake pressure to control the driven wheels such that the wheels are not allowed to freely spin. This is accomplished by first detecting an incipient spin condition indicated by excessive wheel acceleration and/or large negative amounts of wheel slip. Once an incipient spin condition has been detected, the wheel spin control system seeks to reduce the magnitude of wheel slip by reducing engine torque and/or applying brake pressure to the spinning wheel. After this corrective action is taken, the wheel speed begins decreasing. When the incipient spin condition has been alleviated, the wheel spin control system then allows greater engine torque and/or reduces brake pressure at the wheel to provide for greater wheel speed. The orchestration of engine and brake control for the purposes of limiting slip at the driven wheels is well known, and can be found in U.S. Pat. No. 4,762,196 Harada et al and U.S. Pat. No. 4,739,856 Inagaki et al.
When a vehicle begins moving from rest, it is said to be launching. It is during vehicle launch that effective slip control of the driven wheels becomes most critical. Although some amount of slip between the vehicle and the driven wheels is necessary to get the vehicle moving, excessive slip (also called spin) means that the tractive force is not maximized and the vehicle is literally "spinning its wheels". By controlling the speed of the driven wheel so as to maximize the tractive force as translated through the drivetrain to the driven wheels, vehicle launch effectiveness is optimized.
As previously related, the tractive force between the wheel and road surface varies in relation to the road surface conditions. To achieve optimal spin control on any given surface, it is therefore important for the spin control system to accurately determine the coefficient of friction of the operating surface and the corresponding critical slip value. The great majority of known systems have fixed wheel slip set-points which they use to determine whether an incipient spin condition is present. The disadvantage of known systems, therefore, is that these systems do not have an effective means for detecting an incipient spin condition when it is actually occurring, but rather choose fixed slip thresholds to determine raw boundaries of a possible spin condition. For a wheel spin control system to provide for peak efficiency of wheel speed and, hence, wheel slip so as to maximize vehicle launch effectiveness, the system should recognize the particular tractive characteristics of the operating surface and detect an incipient spin condition as it is actually occurring. Ideally, such a system should be able to adapt to changes in road surface coefficient, as well as be able to operate on a split-.mu. surface, without compromising its control accuracy or effectiveness.